The present invention relates generally to steam sterilization and more specifically to a method and apparatus for more effectively controlling and monitoring the amount of residual air in the sterilizing chamber.
It is known that in a steam sterilization process, a saturated steam condition produces the best biocidal kill. Both air entrapment, that is, residual air in the sterilizer chamber, and super heat conditions within the sterilizer result in an unsaturated steam condition, and therefore, a decreased biocidal kill or sterilization rate. For example, a super heat condition within the chamber can result in a premature actuation of various thermal sensing devices which initiate a timed sterilization period. It is known that a moderate amount of super heat, say between 5.degree. to 10.degree.F, results in an insignificant change in the effectiveness of the sterilization process. Compensating for the effects of super heat within this range is easily accomplished by the prudent placement of the thermal sensing devices within the sterilizing chamber or otherwise protecting the devices so that there is a delay in the sensing of the super heat condition.
It is most desirable to control and monitor the steam sterilizer chamber conditions with pressure operated devices. This is because the chamber pressure is equal throughout and consequently a pressure sensing device does not have to be prudently placed to avoid being actuated by super heat. Further, pressure devices do not exhibit the large thermal lag of temperature sensing devices and such devices are much easier to calibrate and maintain. However, the primary drawback of pressure operated devices, is that the proper operation of such devices in a steam sterilization process is contingent upon the adequate removal of air from the chamber.
Residual or entrapped air in the sterilizing chamber results in the inaccurate operation of the pressure operated controllers and/or indicators in the sterilizer due to the effects of the partial pressure of air. The biological effects of air entrapment also have been reported to be noticeable when the amount of air within the sterilizing chamber exceeds about 1% by volume. While the problem of super heat can be overcome as discussed above, the amount of air remaining in the sterilizer chamber is not readily measurable by pressure-temperature correlation and should the amount of residual air exceed 1 percent, the resulting air and steam condition could adversely effect the sterilization process.
The method and apparatus of the present invention provides for a continuous sampling of the environment within the sterilizing chamber to test for residual air and then initiating the timed sterilizing cycle when the residual air has reached a predetermined low limit. During the sterilizing cycle, the readings of any pressure sensors within the chamber are thus more accurate as there is no correction necessary due to the partial pressure of air within the chamber.